To continuously level thin metal strips two processes are known: With stretch leveling a free length of the metal strip is tensioned between two roller pairs with a force exceeding its yield limit so as to be plastically lengthened. With stretch bending a strip pretensioned below its yield limit is deflected about rollers of small diameter so that the bending subjects the outer surface to an additional tension. The bending tension combined with the prestress creates a plastic lengthening of the strip that takes place on the upper and lower faces as the strip is looped back and forth.
Stretch bending is likely to mar the surface from the many contacts with the small rollers that are normally made of steel. This method is thus disadvantageous when strips with a high surface quality are needed, e.g. for lithographic purposes.
German patent 3,525,343 describes the advantages and disadvantages of the two leveling methods and advises for thin metal strips, e.g. aluminum strips 0.1 to 0.33 mm thick, a combination of the two methods.
German patent 3,912,676 describes an apparatus of this type that works according to the principles of pure stretch leveling. Between a set of torque-controlled braking rollers and a set of torque-controlled driven rollers is a rotation-controlled pair of leveling rollers by means of which the strip is subjected to the necessary stretching to plastically deform it.
It is recognized--as described in German patent 3,912,676--that with each lengthening, whether elastic or plastic, there is a transverse contraction. With metal strips the ratio of longitudinal stretch to transverse contraction, the Poisson number, is about 0.3; that is the width change is about one-third of the length change. It has been proven that the planarity during leveling is damaged when the transverse shrinkage is impeded by the strip sticking to the retaining rollers. Only in the free stretches between two succeeding rollers can the strip draw together unhindered transversely.